function change_pic(){
    var imgObj = document.getElementById("ferry");
    var Flag=(imgObj.getAttribute("src",2)=="images/ferry.jpg")
    imgObj.src=Flag?"images/ferry_ps.jpg":"images/ferry.jpg";
}

function change_pic1(){
    var imgObj = document.getElementById("sky");
    var Flag=(imgObj.getAttribute("src",2)=="images/sky.jpg")
    imgObj.src=Flag?"images/sky_ps.jpg":"images/sky.jpg";
}

function change_pic2(){
    var imgObj = document.getElementById("flowers");
    var Flag=(imgObj.getAttribute("src",2)=="images/flowers.JPG")
    imgObj.src=Flag?"images/flowers_ps.jpg":"images/flowers.JPG";
}